Motion Tracking: No Camera for Frame X
Enable the keyframe box, and then set the range of keyframes you want to use for solving in the boxes below (keyframe A and Keyframe B).

Blender will solve the frames it can solve, meaning the frames for which there is a reasonable amount of information to estimate placement in the 3d world (or a low error).  The slected range is used to determine the camera intrinsics and placement.  Blender will then try solve the whole shot as marked in the beginning and end frame of the scene, but using the information gathered in the marked section of the shot.
For more information on motion tracking read this post:
How can I get better results when doing camera motion tracking?